Charge decay measurement systems and methods

ABSTRACT

Various approaches to can be used to interrogate a surface such as a surface of a layered semiconductor structure on a semiconductor wafer. Certain approaches employ Second Harmonic Generation and in some cases may utilize pump and probe radiation. Other approaches involve determining current flow from a sample illuminated with radiation. Decay constants can be measured to provide information regarding the sample. Additionally, electric and/or magnetic field biases can be applied to the sample to provide additional information.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit under 35 U.S.C. §119(e) of U.S.Provisional Application No. 61/980,860, filed on Apr. 17, 2014, titled“WAFER METROLOGY TECHNOLOGIES,” which is incorporated by referenceherein in its entirety, including but not limited to each of theSections I, II, III, and IV, which are each incorporated herein byreference in their entirety.

FIELD

The subject filing relates to systems for based wafer inspection,semiconductor metrology, materials characterization, surfacecharacterization and/or interface analysis.

BACKGROUND

In nonlinear optics, light beam input(s) are output as the sum,difference or harmonic frequencies of the input(s). Second HarmonicGeneration (SHG) is a non-linear effect in which light is emitted from amaterial at a reflected angle with twice the frequency of an incidentsource light beam. The process may be considered as the combining of twophotons of energy E to produce a single photon of energy 2E (i.e., theproduction of light of twice the frequency (2ω) or half the wavelength)of the incident radiation.

A survey of scientific investigations in which the SHG technique hasbeen employed is provided by, “Optical Second-Harmonic Generation fromSemiconductor Surfaces” by T. F. Heinz et al., Published in Advances inLaser Science III, edited by A. C. Tam, J. L. Cole and W. C. Stwalley(American Institute of Physics, New York, 1988) p. 452. As reviewed, theSHG process does not occur within the bulk of materials exhibiting acenter of symmetry (i.e., in inversion or centrosymmetric materials).For these materials, the SHG process is appreciable only at surfacesand/or interfaces where the inversion symmetry of the bulk material isbroken. As such, the SHG process offers a unique sensitivity to surfaceand interface properties.

So-understood, the SHG effect is described in U.S. Pat. No. 5,294,289 toHeinz et al. Each of U.S. Pat. No. 5,557,409 to Downer, et al., U.S.Pat. Nos. 6,795,175; 6,781,686; 6,788,405; 6,819,844; 6,882,414 and7,304,305 to Hunt, U.S. Pat. No. 6,856,159 to Tolk, et al. and U.S. Pat.No. 7,158,284 to Alles, et al. also describe other approaches or “tools”that may be employed. Yet, the teachings of these patents appear not tohave overcome some of the main obstacles to the adoption of SHG as anestablished technique for use in semiconductor manufacturing andmetrology.

SUMMARY

To date, there has been limited adoption of SHG-based metrology tools.It is believed that this fact stems from an inability of existingsystems to make distinctions between detected interfacial properties. Inother words, while existing SHG techniques offer means of determininglocation and presence of interfacial electrically active anomalies,their methods rely on relative measurements and are not practically ableto parse between electrically active anomaly types (e.g., getteredcontaminants such as copper vs. bond voids) and/or to quantify detectedcontaminants.

However, the subject systems and methods variously enable capturing thequantitative information for making the determinations required for suchactivity. In these systems and methods, after charging a wafer samplewith optical electro-magnetic radiation (at a specific site with apulsed laser or with a flash lamp or other electro-magnetic energysource or light source or other means) a plurality of measurements aremade to monitor transient electric field decay associated withheterointerfaces controlling the decay period.

Using decay curve data generated and characterized with multiple points,spectroscopic parameters of an anomaly or problem at a sample site canbe determined such that differentiation and/or quantification of defecttype or contaminant(s) is possible. In all, the decay dependent data iscollected and used to provide systems by which charge carrier lifetimes,trap energies and/or trapped charge densities may be determined in orderthat defects and contaminants can be discerned or parsed from oneanother, for species differentiation if a contaminant is detected and/orfor contaminant quantification if detected

Such activity is determined on a site-by-site basis with the selectedmethodology typically repeated to scan an entire wafer or other materialsample or region thereof. As for the computer processing required toenable such determination, it may occur in “real time” (i.e., during thescanning without any substantial delay in outputting results) or viapost-processing. However, in various embodiments, control software canrun without lag in order to provide the precise system timing to obtainthe subject data per methodology as described below.

Optionally, sample material charge-up is monitored in connection withSHG signal production. In which case, the information gained via thissignal may be employed in material analysis and making determinations.

In any case, system embodiments may include an ultra-short pulse laserwith a fast shutter operating in the range of 10² seconds to picosecond(10⁻¹² seconds) range. Such systems may be used to monitor SHG signalgeneration at a sample site from surface and buried interfaces of thinfilm materials after the introduction of a plurality of short blockingintervals. These intervals may be timed so as to monitor the field decayof interest.

The subject systems may also include an optical line delay. The delayline may be a fiber-based device, especially if coupled with dispersioncompensation and polarization control optics. Alternatively, the delayline may be mirror-based and resemble the examples in U.S. Pat. No.6,147,799 to MacDonald, U.S. Pat. No. 6,356,377 to Bishop, et al. orU.S. Pat. No. 6,751,374 to Wu, et al. In any case, the delay is used inthe system in order to permit laser interrogation of the material in thepicosecond (10⁻¹² second) to femtosecond (10⁻¹⁵ second) and, possibly,attosecond (10⁻¹⁸ second) ranges. Such interrogation may be useful indetecting multiple charge decay-dependent data points along a singledecay curve.

The subject methods include one that involves measuring an SHG signalfor decay data points acquired after successive charge-up events. Theconditions for obtaining a SHG signal may be different at each charge-upevent. Additionally, the time interval between successive charge-upevents may be different. In this method, the multiple data points (atleast two but typically three or more) can be correlated and expressedas a single composite decay curve. Another method employs minimallydisruptive (i.e., the radiation used to produce the SHG signal does notsignificantly recharge the material) SHG signal interrogation eventsafter a single charging event.

Yet another method for determining transient charge decay involvesmeasuring discharge current from the sample material (more accurately,its structures that were charged by optical radiation). The timedependence (kinetics) of this signal may then be treated in the same wayas if SHG sensing had been employed. Further, as above, such sensing maybe done in the span of one decay interval and/or over a plurality ofthem following charge to a given level. In any case, electrode-specifichardware for such use is detailed below.

Regarding charge or charging level, this may be taken to a point ofapparent saturation when charge dynamics are observed in standard lineartime or against a log time scale. Per above, the subject methodologiesoptionally observe, record and analyze charging kinetic as this mayyield important information.

For successive charge/interrogation events, if an initial charge stateof a sample is measured and the saturation level is not far from theinitial charge state, the system may omit further or subsequentcharacterization. In this context, what may be regarded as “not far” maymean about 1% to about 10% of charge increase versus the initial chargestate to be determined by learning when the subject tool is used for agiven time of sampling.

Stated otherwise, so-called “saturation” is a relative term. Using alinear time scale, material will appear saturated very quickly. But ifan SHG signal intensity associated with charging is observed in logscale from 10-100 seconds, it can be observed that the later part ofsaturation occurs with a different time constant and is relatively moregradual or time-consuming. Thus, while examples of the methodologyprovided herein discuss charging to saturation, the delay and othertiming may be regarded as occurring with respect to apparent saturation.Rather than waiting the full amount of time for 100% saturation, as thismay be unnecessarily time consuming to reach, instead, the instrumentmay delay until the time it takes to get to apparent saturation or thetime in which can extract important parameters, regardless of how longit takes for full saturation.

Further, it is to be understood that when monitoring the amount ordegree of charge-up toward saturation (e.g., in connection with SHGmonitoring), the subject methods and systems may operate with chargeand/or re-charging levels at less than saturation (as discussed above)while still yielding meaningful decay curve information. Without suchmeasurement, however, when approximate saturation is a known parameter(e.g., by experience with the subject tool with a given material) chargeto saturation is employed as the target level.

Notably, various interfacial material properties may also be determinedusing laser beam blocking or delay as further described in the portionof U.S. Provisional Application No. 61/980,860, filed on Apr. 17, 2014,titled “WAFER METROLOGY TECHNOLOGIES,” referred to as Section III,titled “TEMPERATURE-CONTROLLED METROLOGY,” which is incorporated hereinby reference in its entirety. Introducing a DC bias across the samplebeing tested can also assist in analysis of the material. Employing a DCbias actively changes the initial charge distribution at the interfacesbefore photo-induced voltage has any effect. To do so, the sample beingtested may be mounted atop a conductive chuck which can be used as aground for DC biasing across the sample using sample top surface probes.Other means of introducing induced voltage biases are possible as wellwithout the use of surface probes as further described in the portion ofU.S. Provisional Application No. 61/980,860, filed on Apr. 17, 2014,titled “WAFER METROLOGY TECHNOLOGIES,” referred to as Section IVentitled, “FIELD-BIASED SHG METROLOGY,” which is incorporated herein byreference in its entirety. See also co-pending U.S. patent applicationSer. No. ______, filed Apr. 17, 2015 titled “FIELD-BIASED SECONDHARMONIC GENERATION METROLOGY”, published as U.S. Publication No.______, which is incorporated herein by reference in its entirety.

Also, the subject systems may use a secondary light source in additionto the primary laser involved in blocking-type analysis for charge decaydetermination. Such a set of sources may be employed as a radiationpump/probe combination as further described in the portion of U.S.Provisional Application No. 61/980,860, filed on Apr. 17, 2014, titled“WAFER METROLOGY TECHNOLOGIES,” referred to as Section I entitled, “PUMPAND PROBE TYPE SHG METROLOGY,” which is incorporated herein by referencein its entirety. See also co-pending U.S. patent application Ser. No.______, filed Apr. 17, 2015 titled “PUMP AND PROBE TYPE SECOND HARMONICGENERATION METROLOGY”, published as U.S. Publication No. ______, whichis incorporated herein by reference in its entirety.

All said, invention embodiments hereof include each of the methodologyassociated with the approaches described herein, alone or in combinationwith elements components or features from the referenced co-pendingpatent applications and/or from the disclosure incorporated herein byreference, hardware to carry out the methodology, productions systemsincorporating the hardware and products (including products-by-process)thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The figures diagrammatically illustrate aspects of various embodimentsof different inventive variations.

FIGS. 1A-1C are diagrams of systems embodiments;

FIG. 2 is a chart of system function;

FIGS. 3A and 3B are charts representative of the manner of deliveringsuch function;

FIG. 4 represents system function in a graphical output.

FIGS. 5 and 6 plot SHG interrogation-related method embodiments;

FIGS. 7A-7E plot time dynamics associated with the system in FIG. 1Cthat may be employed in the methods of FIGS. 5 and 6.

FIG. 8 plots a current-based interrogation method for observingtransient electric field decay;

FIGS. 9A and 9B illustrate hardware configurations that may be employedin the method of FIG. 8.

DETAILED DESCRIPTION

FIG. 1A is a diagram of a first system 2100 as may employed inconnection with the subject methodology. Alternative systems 2100′ and2100″ are shown in FIGS. 1B and 1C. Each system includes a primary laser2010 for directing a primary beam 2012 of electro-magnetic radiation ata sample wafer 2020, which sample is held by a vacuum chuck 2030. Thechuck 2030 includes or is set on x- and y-stages and optionally also arotational stage for positioning a sample site 2022 across the waferrelative to where the laser(s) are aimed. A beam 2014 of reflectedradiation directed at a detector 2040 will include an SHG signal. Thedetector may be any of a photomultiplier tube, a CCD camera, a avalanchedetector, a photodiode detector, a streak camera and a silicon detector.The sample site 2022 can include one or more layers. The sample site2022 can comprise a composite substrate including at least two layers.The sample site 2022 can include an interface between two dissimilarmaterials (e.g., between two different semiconductor materials, betweentwo differently doped semiconductor materials, between a semiconductorand an oxide, between a semiconductor and a dielectric material, betweena semiconductor and a metal, between an oxide and a metal, between ametal and a metal or between a metal and a dielectric).

Also common to each of the embodiments is the inclusion of one or moreshutter-type devices 2050. These are employed as described in connectionwith the methodology below. The type of shutter hardware used willdepend on the timeframe over which the laser radiation is to be blocked,dumped or otherwise directed away from the sample site.

An electro-optic blocking device such as a Pockel's Cell or Kerr Cell isused to obtain very short blocking periods (i.e., with switching timeson the order of 10⁻⁹ to 10⁻¹² seconds). For longer blocking timeintervals (e.g., from about 10⁻⁵ seconds and upwards) mechanicalshutters or flywheel chopper type devices may be employed.

However, electro-optic blocking devices will allow a wider range ofmaterials to be tested in accordance with the methods below. A photoncounting system 2044 capable of discretely gating very small timeintervals, typically, on the order of picoseconds to microseconds can beincluded to resolve the time-dependent signal counts.

Hardware is contemplated for pushing the methods into faster-yet timeframes. Namely, as shown in FIG. 1C, the system(s) may include delayline hardware 2060. Beam splitting and switching (or shuttering on/off)between a plurality of set-time delay lines for a corresponding numberof time-delayed interrogation events is possible. However, a variabledelay line may be preferred as offering a single solution for multipletransient charge decay interrogation events on a time frame ranging fromimmediately (although delay of only 10⁻¹² seconds may be required formany methodologies) to tens of nanoseconds after pump pulse. The desireddelay time may even go into the microsecond regime if using a slower,kilohertz repetition laser. And while such hardware is uniquely suitedfor carrying out the subject methodology (both of which methodology andsuch hardware is believed heretofore unknown), it might be put to otheruses as well.

In the implementation illustrated in FIG. 1C, the beam 2012 from thelaser 2010 can be split by a beam splitter 2070 between two opticalpaths. The beam splitter 2070 can split the beam 2012 unequally betweenthe two optical paths. For example, 70% of the energy of the beam 2012can be directed along a first optical path (e.g., as beam 2016) and 30%of the energy of the beam 12 can be directed along a second optical path(e.g., as beam 2018). As another example, 60% of the energy of the beam2012 can be directed along the first optical path and 40% of the energyof the beam 2012 can be directed along the second optical path. As yetanother example, 80% of the energy of the beam 2012 can be directedalong the first optical path and 20% of the energy of the beam 2012 canbe directed along the second optical path. The beam splitter 2070 cancomprise a dielectric mirror, a splitter cube, a metal coated mirror, apellicle mirror or a waveguide splitter. In implementations, where thebeam 2012 includes optical pulses, the beam splitter 2070 can include anoptical component having negligible dispersion that splits the beam 2012between two optical paths such that optical pulses are not broadened. Asindicated by the double-arrow in FIG. 1C, the path of an “interrogation”beam 2016 taken off a beam splitter 2070 from primary beam 2012 can belengthened or shortened to change its arrival timing relative to a“pump” beam 2018 wherein each of the beams are shown directed or aimedby various mirror elements 2072. Another approach (mentioned above)employs fiber optics in the optical delay component and/or other opticalpathways (e.g., as presented in U.S. Pat. No. 6,819,844 incorporatedherein by reference in its entirety for such description).

The output from the detector 2040 and/or the photon counting system 2044can be input to an electronic device 2048 (see, e.g., FIGS. 1A and 1B).The electronic device 2048 can be a computing device, a computer, atablet, a microcontroller or a FPGA. The electronic device 2048 includesa processor or processing electronics that may be configured to executeone or more software modules. In addition to executing an operatingsystem, the processor may be configured to execute one or more softwareapplications, including a web browser, a telephone application, an emailprogram, or any other software application. The electronic device 2048can implement the methods discussed herein by executing instructionsincluded in a machine-readable non-transitory storage medium, such as aRAM, ROM, EEPROM, etc. The electronic device 2048 can include a displaydevice and/or a graphic user interface to interact with a user. Theelectronic device 2048 can communicate with one or more devices over anetwork interface. The network interface can include transmitters,receivers and/or transceivers that can communicate over wired orwireless connections.

Another potential aspect of system 2100″ concerns the manner in whichthe initial beam splitter works. Namely, the split may be unequal (e.g.,70-30%, 80-20%, 60-40% or any range therebetween, such as between 60-90%in one path and between 40-10% in another path as well as outside theseranges), sending a majority of the power in the pump beam, and aminority in the probe beam. For example, the split may be 60-70% and40-30%, for the pump and probe, respectivley, 70-80% versus 30-20% forthe pump and probe, respectively, 80-90% versus 20-10%, for the pump andprobe respectively, or 90-99.999% versus 10-0.001%, for the pump andprobe respectively. In different embodiments, the probe beam could bebetween 0.001% to 49.99% while the pump beam could be between 50.001%and 99.999%, for example. The sum of the two beams may be 100% orapproximate thereto. The split may be determined by the particularmaterial system being characterized in some cases. The value (at leastin part) of doing so may be to help facilitate methods such as shown inFIGS. 5 and 6 in which the power involved in SHG interrogationsubsequent to material charging is desirably reduced or minimized asdiscussed below. Still another aspect is that the pump and probe beamsare brought in at different angles. Such an approach facilitatesmeasuring pump and probe SHG responses separately. In such cases, twodetectors may be advantageously employed with one for each reflectedbeam path.

Various other optional optics distinguish the embodiments shown. Forexample, embodiments 2100 and 2100′ are shown including a dichroicreflective or refractive filter 2080 for selectively passing the SHGsignal coaxial with reflected radiation directly from the laser 2010.Alternatively, a prism may be employed to differentiate the weaker SHGsignal from the many-orders-of-magnitude-stronger reflected primarybeam. However, as the prism approach has proved to be very sensitive tomisalignment, a dichroic system as referenced above may be preferred.Other options include the use of diffraction grating or a Pellicle beamsplitter. As shown in system 2100, an optical bundle 2082 of focusingand collimating/collimation optics may be provided. As shown in system2100′, a filter wheel 2084, zoom lens 2086 and/or polarizers 2088 may beemployed in the system(s). Also, an angular (or arc-type) rotationaladjustment (with corresponding adjustment for the detector 2040 andin-line optical components) as shown in system 2100′ may be desirable.An additional radiation source 2090 (be it a laser illustrated emittinga directed beam 2092 or a UV flash lamp emitting a diverging oroptically collimated or a focused pulse 2094) may also be incorporatedin the system(s) to provide such features as referenced above inconnection with the portion of U.S. Provisional Application No.61/980,860, filed on Apr. 17, 2014, titled “WAFER METROLOGYTECHNOLOGIES,” referred to as Section I entitled “PUMP AND PROBE TYPESHG METROLOGY,” which is incorporated herein by reference in itsentirety and/or initial charging/saturation in the methods below. Seealso co-pending U.S. patent application Ser. No. ______, filed Apr. 17,2015 titled “PUMP AND PROBE TYPE SECOND HARMONIC GENERATION METROLOGY”,published as U.S. Publication No. ______, which is incorporated hereinby reference in its entirety.

In these systems, laser 10 may operate in a wavelength range betweenabout 700 nm to about 2000 nm with a peak power between about 10 kW and1 GW, but delivering power at an average below about 100 mW. In variousembodiments, average powers between 10 mW and 10 W should be sufficient.Additional light source 2090 (be it a another laser or a flash lamp) mayoperate in a wavelength range between about 80 nm and about 800 nmdelivering an average power between about 10 mW and 10 W. Values outsidethese ranges, however, are possible.

Regarding other system options, since an SHG signal is weak compared tothe reflected beam that produces it, it may be desirable to improve thesignal-to-noise ratio of SHG counts. As photon counting gate timesdecrease for the blocking and/or delay processes described herein,improvement becomes even more useful. One method of reducing noise thatmay be employed is to actively cool the detector. The cooling candecreases the number of false-positive photon detections that aregenerated randomly because of thermal noise. This can be done usingcryogenic fluids such as liquid nitrogen or helium or solid statecooling through use of a Peltier device. Others areas of improvement mayinclude use of a Marx Bank Circuit (MBC) as relevant to shutter speed.

These improvements may be applied to any of the systems in FIGS. 1A-1C.Likewise, any or all of the above features described above in connectionwith systems 2100 and 2100′ may be incorporated in system 2100″. Indeeda, mix-and-match of features or components is contemplated between allof the systems.

With such systems running the subject methodology, variousdeterminations can be made not heretofore possible using laser-blockingand/or delay related techniques. FIG. 2 illustrates a process map ordecision tree 2200 representing such possibilities. Namely, a so-calledproblem 2210 that is detected can be parsed between a defect 2210(extended defects such as bond voids or dislocations, Crystal OriginatedParticle (COP) or the like) and a contaminant 2220 (such as copperinclusion or other metals in point defect or clustered forms). In termsof a defect, the defect type 2222 and/or a defect quantification 2224determination (e.g., in terms of density or degree) can also be made. Interms of a contaminant, the contaminant species or type 2232 and/or acontaminant quantification 2234 determination can be made. Such parsingbetween defect and contaminant and identification of species may beperformed in connection with determining charge carrier lifetimes, trapenergies, trap capture cross-section and/or trap densities thencomparing these to values in look-up tables or databases. Essentiallythese tables or databases include listings of properties of the materialas characterized by the subject methods, and then matching-up the statedproperties with entries in a table or database that correspond toparticular defects or contaminants.

Trap capture cross-section and trap density may be observed inconnection with, optionally, detected charging kinetics. As fordetermining charge carrier lifetimes and trap energies, the followingequation based on work by I. Lundstrom, provides guidance:

$\tau = {\tau \text{?}\exp \left\{ {\frac{4}{3\hslash}{\sqrt{2{em}\text{?}}\left\lbrack {\varphi_{\tau}^{3/2} - {{\left( {\varphi_{\tau}^{3/2} - \left( {\varphi_{r} - {E_{ox}d_{\tau}}} \right)^{3/2}} \right\rbrack/E_{ox}}\text{?}\text{indicates text missing or illegible when filed}}}\mspace{250mu} \right.}} \right.}$

where τ is the tunneling time constant for the tunneling mechanism ofthe trap discharge, φ, denotes the trap energy, E_(ON) denotes thestrength of the electric filed at the interface and the remainingequation variables and context are described at I. Lundstrom, JAP, v.43, n. 12, p. 5045, 1972 which subject matter is incorporated byreference in its entirety. Further modeling and calculation options maybe appreciated in reference to the portion of U.S. ProvisionalApplication No. 61/980,860, filed on Apr. 17, 2014, titled “WAFERMETROLOGY TECHNOLOGIES,” referred to Section III, titled“TEMPERATURE-CONTROLLED METROLOGY,” which is incorporated herein byreference in its entirety.

In any case, the decay curve data obtained by the subject sampleinterrogation can be used to determine the parameters of trap energy andcharge carrier lifetime by use of physical models and relatedmathematics. Representative sets of curves 2300, 2300′ such as thosepictured in FIGS. 3A and 3B may be calculated (where FIG. 3B highlightsor expands a section of the data from FIG. 3A) from the equation above.

These curves demonstrate the relationship between time constant(vertical axis) and dielectric thicknesses (horizontal axis) fordifferent trap or barrier energies. The vertical axis includes theultrafast time scales of down to nanoseconds (1E-9 s)). The horizontalaxes are tunneling distances (or dielectric thickness, both terms beinggenerally equivalent in this example). The different curves are lines ofconstant barrier energy. For example, in FIG. 3B, an electron caught ina trap with an energy depth of the listed barrier energy of 0.7 eV wouldexhibit a detrapping time constant of about 1E-5 seconds if thedielectric thickness was 40 Angstroms.

Further modeling with Poisson/Transport solvers can be used to determinetrap density in MOS-like structures and more exotic devices using chargecarrier lifetimes and known trap energies. Specifically, thephoto-injected current due to femto-second optical pulses induces burstsof charge carriers which reach the dielectric conduction band. Theaverage value of this current can be related to carrier concentrationand their lifetimes in the regions. The E-field across the interface isthe proxy by which SHG measures these phenomena.

In the plot of FIG. 3A, it can be observed (see dashed lines) that 20Angstrom of oxide has 1 msec discharge time constant for a trap havingan energy of about 3 eV To relate the plots to an example of use in thesubject system, suppose a 20 Angstrom oxide is interrogated afterblocking laser excitation. As shown in FIG. 3B (see highlighted box) theresult will be observable current from 1 ρsec to about 1 msec and thenall the current dies out.

The decay curves discussed in this application can be a product ofmultiple processes (e.g., charge relaxation, charge recombination, etc.)from traps having different energies and differentrelaxation/recombination time constants. Nevertheless, in variousembodiments, the decay curves can be generally expressed by anexponential function f(t)=Aexp(−λt)+B, where A is the decay amplitude, Bdenotes the baseline offset constant and λ denotes a decay constant.This general exponential function can be used to approximatelycharacterize the “extent of decay” from experimentally obtained decaydata curve. In various embodiments, it is possible to use the half-lifet_(1/2), average lifetime τ, and decay constant λ, to characterize theextent of decay for a decay curve (obtained experimentally or bysimulation). For example, the parameters A, B, and λ, can be obtainedfrom the decay data points that are obtained experimentally as discussedbelow. An average lifetime τ can then be calculated from the parametersA, B, and λ, using theory of radioactive decay as a way of settingbenchmarks for what is qualitatively called partial, or full-decay. Forexample, in some embodiments, τ can be given by the equation(t_(1/2))/(In(2)).

In various implementations, the charge state can be considered to havefully decayed after a time span of three average lifetimes τ, whichcorresponds to ˜95% decay from full saturation. Partial decay can beexpressed in terms of signal after a certain number of average lifetimesτ have elapsed.

In operations, the systems determine parameters (e.g., carrierlifetimes, trap energies, trapping cross-section, charge carrierdensity, trap charge density, carrier injection threshold energy, chargecarrier lifetime, charge accumulation time, etc.) based at least in parton the subject methodology on a point-by-point basis on a portion (e.g.,die size portion) of the wafer or an entire wafer. An entire wafer(depending on the material, surface area, and density of scan desired)can often be scanned in less than about 10 minutes, with theseparameters determined for each point scanned. In various embodiments, alocation of the wafer can be scanned in a time interval between about100 milliseconds and about 3 seconds. For example, a location of thewafer can be scanned in about 950 milliseconds.

A matrix of data containing the spatial distributions of the parametersdetermined can be plotted as individual color-coded heat maps or contourmaps for each parameter, as a means for quantitative inspection,feedback and presentation. FIG. 4 illustrates one such map 2400. Itdepicts how a defect 2402 may be portrayed. But it is possible to showany of the further refined subject matters in FIG. 2. Once quantitativedata has been obtained, providing such output is merely a matter ofchanging the code in the plotting program/script.

Such information and/or other information treated below may be shown ona computer monitor or dedicated system display and/or it may be recordedfor later reference or use for analysis on digital media. In addition,each wafer spatial distribution can be cross-correlated by referencingwith ellipsometry data to correct for layer thickness variability andcross-calibrated with independent contamination characterization dataobtained, for example, by Total Reflection X-ray Fluorescence (TXRF),Time of Flight Secondary Ion Mass Spectroscopy (TOF-SIMS) and the like.These initial or corrected spatial distributions can then be compared tothose from wafers known to be within specification, to determine if thesamples in question have any defects or problematic features whichwarrant further testing. In general, however, it is desirable to uselow-cost SHG and other methods hereof calibrated with, by or againstslow and expensive direct methods like TXRF, etc.

Human decisions may be employed (e.g., in inspecting a generated heatmap 2400) initially in determining the standard for what is anacceptable or unsatisfactory wafer, until the tool is properlycalibrated to be able to flag wafers autonomously. For awell-characterized process in a fab, human decisions would then onlyneed to be made to determine the root cause of any systemic problem withyields, based on the characteristics of flagged wafers.

However implemented, FIG. 5 provides a plot 2500 illustrating a firstmethod embodiment hereof that may be used in making such determinations.This method, like the others discussed and illustrated below relies oncharacterizing SHG response with multiple shutter blocking events inwhich interrogation laser is gated for periods of time.

In this first example, a section of a sample to be interrogated ischarged (typically by a laser) to saturation. In this example, a singlesource is used to generate as pump beam and probe beam, althoughseparate pump and probe sources can be used in other embodiments. Duringwhich time, the SHG signal may be monitored. The saturation level may beknown by virtue of material characterization and/or observing asymptoticbehavior of the SHG signal intensity associate with charging (I_(ch)).Upon (or after) reaching saturation, the electromagnetic radiation fromthe laser (pump beam) is blocked from the sample section. The laser(probe beam) is so-gated for a selected period of time (t_(b11)). Aftergating ceases, an SHG intensity measurement (I_(dch1)) is made with thelaser (probe beam) exposing the surface, thus observing the decay ofcharge at a first discharge point. After charging the material section(with the pump beam) to saturation again over a period of time (t_(ch)),a second blocking event occurs for a time (t_(b12)) different than thefirst in order to identify another point along what will become acomposite decay curve. Upon unblocking the laser (probe beam), SHGsignal intensity (I_(dchs2)) is measured again. This reduced signalindicates charge decay over the second gating event or blockinginterval. Once-again charged to saturation by the laser (pump beam), athird differently-timed blocking event (t_(b13)) follows and subsequentSHG interrogation and signal intensity measurement (I_(mo)) is made fora third measurement of charge decay in relation to SHG intensity.

Although in the above example, the sample is charged to a saturationlevel, in other examples, the sample can be charged to a charge levelbelow saturation. Although in the above example, the three blockingtimes t_(b11), t_(b12) and t_(b13) are different, in other examples, thethree blocking times t_(b11), t_(b12) and t_(b13) can be the same. Invarious examples, the sample can be charged to a charging levelinitially and the SHG intensity measurement (I_(dch1)), (I_(dch2)) and(I_(dch3)) can be obtained at different time intervals after the initialcharging event.

As referred to above, these three points (corresponding to I_(dch1),I_(dch2) and I_(dch3)) can be used to construct a composite charge decaycurve. It is referred to herein as a “composite” curve in the sense thatits components come from a plurality of related events. And while stillfurther repetition (with the possibility of different gating timesemployed to generate more decay curve data points or the use ofsame-relation timing to confirm certainty and/or remove error frommeasurements for selected points) may be employed so that four or moreblock-then-detect cycles are employed, it should be observed that as fewas two such cycles may be employed. Whereas one decay-related data pointwill not offer meaningful decay curve characterization, a pair defininga line from which a curve may be modeled or extrapolated from to offersome utility, whereas three or more points for exponential decay fittingwill yield an approximation with better accuracy. Stated otherwise, anysimple (e.g., not stretched by dispersive transport physics) decaykinetics has a general formula: Measurable (t)=M₀*exp(−t/tau) so to findtwo unknown parameter M₀ and tau at least 2 points are needed assumingthis simple kinetics. In dispersive (i.e., non-linear) kinetics it isdesirable to measure as many point as possible to extract (n−1)-ordercorrection parameters if n-points are measured and then apply a modelappropriate for that order of approximation. Also, that set ofmeasurements is to be measured for different electric fields (E) to bereal practical and precise with the tau to assign it for a certain typeof defects.

The method above can provide parameter vs. time (such as interfacialleakage current or occupied trap density v. time) kinetic curve byobtaining measurements at a few time points. A time constant (τ) can beextracted from the parameter vs. time kinetic curve. The time constantcan be attributed to a time constant characteristic for a certain typeof defect.

In any case, the decay-dependent data obtained may be preceded (as inthe example) by SHG data acquisition while saturating the material withthe interrogation (or probe) laser. However, charging will notnecessarily go to saturation (e.g., as noted above). Nor will themeasurement necessary be made prior to the blocking of a/the charginglaser. Further, the charging will not necessarily be performed with theinterrogation/probe laser (e.g., see optional pump/probe methodologycited above).

Regardless, after the subject testing at one sample site, the samplematerial is typically moved or indexed to locate another section for thesame (or similar) testing. In this manner, a plurality of sections oreven every section of the sample material may be interrogated andquantified in scanning the entire wafer as discussed above.

FIG. 6 and plot 2600 illustrate an alternative (or complimentaryapproach) to acquiring charge decay related data by scanning is shown inplot 2600. In this method, after charging to saturation a/the firsttime, continuous (or at least semi-continuous) discharge over multipleblocking time intervals (t_(b11), t_(b12), t_(b13)) is investigated bylaser pulses from an interrogation or a probe laser measuring differentSHG intensities (I_(dch1), I_(dch2), I_(dch3)). The intensity and/orfrequency of the laser pulses from the interrogation/probe laser areselected such that the average power of the interrogation/probe laser isreduced to avoid recharging the material between blocking intervalswhile still obtaining a reasonable SHG signal. To do so, as little asone to three laser pulses may be applied. So-reduced (in number and/orpower), the material excitation resulting from the interrogation orprobe laser pulses may be ignored or taken into account by calibrationand or modeling considerations.

In various embodiments, a separate pump source can be used for charging.However, in some embodiments, the probe beam can be used to charge thesample.

In any case, the delay between pulses may be identical or tuned toaccount for the expected transient charge decay profile or for otherpractical reason. Likewise, while the delay is described in terms of“gating” or “blocking” above, it is to be appreciated that the delay maybe produced using one or more optical delay lines as discussed above inconnection with FIG. 1C. Still further, the same may hold true for theblocking/gating discussed in association with FIG. 5.

Further, as above, the method in FIG. 6 may be practiced with variousmodifications to the number of blocking or delay times or events. Also,SHG signal may or may not be measured during charge to saturation.Anyway, the method in FIG. 6 may be practiced (as illustrated) such thatthe final gating period takes the SHG signal to null. Confirmation ofthis may be obtained by repeating the method at the same site in a modewhere charging intensity (I_(ch)) is measured or by only observing theSHG signal in (re)charging to saturation.

FIGS. 7A-7E are instructive regarding the manner in which the subjecthardware is used to obtain the decay-related data points. FIG. 7Aprovides a chart 2700 illustrating a series of laser pulses 2702 inwhich intermediate or alternating pulses are blocked by shutter hardware(e.g., as described above) in a so-called “pulse picking” approach. Overa given time interval, it is possible to let individual pulses through(indicated by solid line) and block others (as indicated by dashedline).

FIG. 7B provides a chart 2710 illustrating the manner in whichresolution of a blocking technique for SHG investigation can be limitedby the repetition (rep) rate of the probe laser. Specifically, whenpresented with a decay curve like decay curve 2712 it is possible toresolve the time delay profile with blocking of every other pulse usinga pulsed laser illustrated to operate at the same time scale as in FIG.7A. However, a shorter curve 2714 cannot be resolved or observed undersuch circumstances. As such, use optical delay stage(s) can offeradditional utility.

Accordingly, chart 2720 in FIG. 7C illustrates (graphically and withtext) how blocking and introducing a delay with respect to a referencetime associated with charging the sample can offer overlapping areas ofusefulness, in terms of the decay time of the curve relative to the reprate of the laser. It also shows how there are short time ranges whenonly delay stages would allow interrogation of the decay curve, andlonger time ranges when only blocking the pumping and/or the probingbeam would be practical.

FIGS. 7D and 7E further illustrate the utility of the combinedblock/delay apparatus. Chart 2730 illustrates exemplary SHG signalsproduced by individual laser pulses 2702. With a delay stage alone, onlythe range (X) between each such pulse may be interrogated by varyingoptical delay. In contrast, additional utility over a range (Y) may beachieved with a system combining a delay stage and blocking or shuttermeans such as a chopper, shutter, or modulator. As illustrated by chart2740, such a system is able to measure decay curves (and theirassociated time constants) in the range from one to several pulse times.

FIG. 8 provides a plot 2800 illustrating a third method embodimenthereof. This embodiment resembles that in FIG. 6, except that dischargecurrent (J_(dch1), J_(dch2), J_(dch3)) is measured at time intervals(e.g., t_(i)=t₀, 2t₀, 3t₀, 7t₀, 10t₀, 20t₀, 30t₀, 70t₀—basicallyaccording to a log time scale vs. linear time—where t₀ is a scaleparameter of about 10⁻⁶ sec or 10⁻³ sec when measurement is started)after charging the material with a laser (optionally monitoring orcapturing its SHG intensity (I_(ch)) signal) or other electro-magneticradiation source and then blocking or otherwise stopping the laserradiation application to the sample, thereby allowing discharge. Thisapproach gives an estimation of the mobile carrier lifetime in thesubstrate by the moment after the e-h-plasma in the substrate is decayedand when the discharge current starts to be seen, thus offering animportant physical parameter of the wafer. And after carrier lifetime isdetermined, the discharge of current can be interpreted in its timedependence (i.e., its kinetics regarding charge decay) in the samemanner as if it were obtained by SHG sensing of discharged charge.

Various embodiments can be used to measure time constant (e.g., fordecay) having a range of values. For example, the time constants canrange between 0.1 femtosecond and 1 femtosecond, 1 femtosecond and 10femtoseconds, 10 femtoseconds and 100 femtoseconds, 100 femtoseconds and1 picosecond, between 1 picosecond and 10 picoseconds, between 10picoseconds and 100 picoseconds, between 100 picoseconds and 1nanosecond, between 1 nanosecond and 10 nanoseconds, between 10nanosecond and 100 nanoseconds, between 100 nanoseconds and 1microsecond, between 1 nanoseconds and 100 microseconds, between 100microseconds and 1 millisecond, between 1 microsecond and 100milliseconds, between 100 microsecond and 1 second, between 1 second and10 seconds, or between 10 second and 100 seconds or larger or smaller.Likewise, time delays (Δ) for example between the probe and pump (orpump and probe) can be, for example, between 0.1 femtosecond and 1femtosecond, 1 femtosecond and 10 femtoseconds, 10 femtoseconds and 100femtoseconds, 100 femtoseconds and 1 picosecond, between 1 picosecondand 10 picoseconds, between 10 picoseconds and 100 picoseconds, between100 picoseconds and 1 nanosecond, between 1 nanosecond and 10nanoseconds, between 10 nanosecond and 100 nanoseconds, between 100nanoseconds and 1 microsecond, between 1 nanoseconds and 100microseconds, between 100 microseconds and 1 millisecond, between 1microsecond and 100 milliseconds, between 100 microsecond and 1 second,between 1 second and 10 seconds, between 10 second and 100 seconds.Values outside these ranges are also possible.

Various physical approaches can be taken in providing a system suitablefor carrying out the method in FIG. 8—which method, notably, may bemodified like those described above. Two such approaches are illustratedin FIGS. 9A and 9B.

Systems 2900 and 2900′ use gate electrodes 2910 and 2920, respectively,made of a conductive material that is transparent in the visible lightrange. Such an electrode may touch a wafer 2020 to be inspected, butneed not as they may only be separated by a minimal distance. In variousimplementations, the electric field in the dielectric can be estimatedby extracting the electrode-dielectric-substrate structure parametersusing AC measurement of the Capacitance-Voltage curve (CV-curve).CV-curve measurement can be done by using a standard CV-measurementsetup available on the market, connected to a material sample in thesubject tool (e.g., the applied voltage is to provide the electric fieldin the dielectric between about 0.1 MV/cm and about 5 MV/cm). The wafermay be held upon a conductive chuck 2030 providing electrical substratecontact. Another alternative construction for a gate electrode would bean ultra-thin Au film or Al film on a glass of 10-30 A thickness whichcan reduce the sensitivity due to absorption of some photons by the thinsemi-transparent metal layer.

However, electrodes 2910 and 2920 present no appreciable absorptionissues (although some refraction-based considerations may arise that canbe calibrated out or may be otherwise accounted for in the system).These electrodes may comprise a transparent conductor gate layer 2930made of a material such as ZnO, SnO or the like connected with anelectrical contact 2932. An anti-reflective top coat 2934 may beincluded in the construction. Gate layer 2930 may be set upon atransparent carrier made 2936 of dielectric (SiO₂) with a thickness(D_(gc)) as shown. In various embodiments, the transparent carriercomprises an insulator that is used as a gate for a noncontact electrodethat may employ for example capacitive coupling to perform electricalmeasurements, similar to those described in the portion of U.S.Provisional Application No. 61/980,860, filed on Apr. 17, 2014, titled“WAFER METROLOGY TECHNOLOGIES,” referred to as Section IV entitled,“FIELD-BIASED SHG METROLOGY,” which is incorporated herein by referencein its entirety. See also co-pending U.S. patent application Ser. No.______, filed Apr. 17, 2015 titled “FIELD-BIASED SECOND HARMONICGENERATION METROLOGY”, published as U.S. Publication No. ______, whichis incorporated herein by reference in its entirety. As the wafer ischarged from the incoming laser radiation, the electric field across oneor more of its interfaces will change, and the layers of the wafershould capacitavely couple with the plates in the electrode similar to aplate capacitor. The charging of the electrode will involve movement ofcharge carriers that will be measured as current.

D_(gc) would be calibrated by measuring CV curve on the semiconductorsubstrate with a non-invasive approach and used in electric field (E)calculation when applied voltage is known. A negligible gap distancebetween the gate and sample can be an air gap. Alternatively theelectrode can be directly in contact with the sample rather than beingseparated by an air gap or dielectric. Accordingly normal CV or IVmeasurements may be performed in various embodiments.

Or given a close refractive index match between water and SiO₂, fillingthe gap with deionized water may be helpful in reducing boundary-layerreflection without any ill effect (or at least one that cannot beaddressed). Deionized (or clean-room grade) water can maintaincleanliness around the electrically sensitive and chemically puresubstrate wafers. Deionized water is actually less conductive thanregular water.

In FIG. 9B, a related construction is shown with the difference beingthe architecture of the carrier or gate-holder 2938. Here, it isconfigured as a ring, optimally formed by etched away in the center andleaving material around the electrode perimeter as produced using MEMStechniques. But in any case, because of the large unoccupied zonethrough with the laser and SHG radiation must pass, it may be especiallydesirable to fill the same with DI water as described above.

Regardless, in the overall electrode 2910, 2920 constructions eachembodiment would typically be stationary with respect to the radiationexciting the material in use. Prior to and after use, the electrodestructure(s) may be stowed by a robotic arm or carriage assembly (notshown).

As describe above, in various embodiments the electrode directlycontacts the wafer to perform electrical measurements such as measuringcurrent flow. However, non-contact methods of measuring current, such asfor example using electrodes that are capacatively coupled with thesample, can also be used.

The systems and methods described herein can be used to characterize asample (e.g., a semiconductor wafer or a portion thereof). For example,the systems and methods described herein can be used to detect defectsor contaminants in the sample as discussed above. The systems andmethods described herein can be configured to characterize the sampleduring fabrication or production of the semiconductor wafer. Thus, thesystems and methods can be used along a semiconductor fabrication linein a semiconductor fabrication facility. The systems and methodsdescribed herein can be integrated with the semiconductorfabrication/production line. The systems and methods described hereincan be integrated into a semiconductor fab line with automated waferhandling capabilities. For example, the system can be equipped with anattached Equipment Front End Module (EFEM), which accepts wafercassettes such as a Front Opening Unified Pod (FOUP) Each of thesecassettes can be delivered to the machine by human operators or byautomated cassette-handling robots which move cassettes from process toprocess along fabrication/production line.

In various embodiments, the system can be configured such that once thecassettes are mounted on the EFEM, the FOUP is opened, and a robotic armselects individual wafers from the FOUP and moves them through anautomatically actuated door included in the system, into a light-tightprocess box, and onto a bias-capable vacuum chuck. The chuck may bedesigned to fit complementary with the robotic arm so that it may laythe sample on top. At some point in this process, the wafer can be heldover a scanner for identification of its unique laser mark.

Accordingly, a system configured to be integrated in a semiconductorfabrication/assembly line can have automated wafer handling capabilityfrom the FOUP or other type of cassette; integration with an EFEM asdiscussed above, a chuck designed in a way to be compatible with robotichandling, automated light-tight doors which open and close to allowmovement of the robotic wand/arm and software signaling to EFEM forwafer loading/unloading and wafer identification.

As described above, each of Sections I, II, III, and IV of U.S.Provisional Application No. 61/980,860, filed on Apr. 17, 2014, titled“WAFER METROLOGY TECHNOLOGIES,” are incorporated herein by reference intheir entirety. Similarly, co-pending patent applications (i) U.S.patent application Ser. No. ______, filed Apr. 17, 2015 titled “PUMP ANDPROBE TYPE SECOND HARMONIC GENERATION METROLOGY”, published as U.S.Publication No. ______, and (ii) U.S. patent application Ser. No.______, filed Apr. 17, 2015 titled “FIELD-BIASED SECOND HARMONICGENERATION METROLOGY”, published as U.S. Publication No. ______, areeach incorporated herein by reference in their entirety. PCT ApplicationNo. PCT/US2015/026263, filed Apr. 16, 2015 titled “WAFER METROLOGYTECHNOLOGIES” is also incorporated herein by reference in its entirety.Accordingly, features from the disclosure of any of these documentsincorporated by reference may be combined with any features recitedelsewhere herein.

Variations

Exemplary invention embodiments, together with details regarding aselection of features have been set forth above. As for other details,these may be appreciated in connection with the above-referenced patentsand publications as well as is generally known or appreciated by thosewith skill in the art. The same may hold true with respect tomethod-based aspects of the invention in terms of additional acts ascommonly or logically employed. Regarding such methods, includingmethods of manufacture and use, these may be carried out in any order ofthe events which is logically possible, as well as any recited order ofevents. Furthermore, where a range of values is provided, it isunderstood that every intervening value, between the upper and lowerlimit of that range and any other stated or intervening value in thestated range is encompassed within the invention. Also, it iscontemplated that any optional feature of the inventive variationsdescribed may be set forth and claimed independently, or in combinationwith any one or more of the features described herein.

Though the invention embodiments have been described in reference toseveral examples, optionally incorporating various features, they arenot to be limited to that which is described or indicated ascontemplated with respect to each such variation. Changes may be made toany such invention embodiment described and equivalents (whether recitedherein or not included for the sake of some brevity) may be substitutedwithout departing from the true spirit and scope hereof.

The various illustrative processes described may be implemented orperformed with a general purpose processor, a Digital Signal Processor(DSP), an Application Specific Integrated Circuit (ASIC), a FieldProgrammable Gate Array (FPGA) or other programmable logic device,discrete gate or transistor logic, discrete hardware components, or anycombination thereof designed to perform the functions described herein.A general purpose processor may be a microprocessor, but in thealternative, the processor may be any conventional processor,controller, microcontroller, or state machine. The processor can be partof a computer system that also has a user interface port thatcommunicates with a user interface, and which receives commands enteredby a user, has at least one memory (e.g., hard drive or other comparablestorage, and random access memory) that stores electronic informationincluding a program that operates under control of the processor andwith communication via the user interface port, and a video output thatproduces its output via any kind of video output format, e.g., VGA, DVI,EIDMI, DisplayPort, or any other form.

A processor may also be implemented as a combination of computingdevices, e.g., a combination of a DSP and a microprocessor, a pluralityof microprocessors, one or more microprocessors in conjunction with aDSP core, or any other such configuration. These devices may also beused to select values for devices as described herein.

The steps of a method or algorithm described in connection with theembodiments disclosed herein may be embodied directly in hardware, in asoftware module executed by a processor, or in a combination of the two.A software module may reside in Random Access Memory (RAM), flashmemory, Read Only Memory (ROM), Electrically Programmable ROM (EPROM),Electrically Erasable Programmable ROM (EEPROM), registers, hard disk, aremovable disk, a CD-ROM, or any other form of storage medium known inthe art. An exemplary storage medium is coupled to the processor suchthat the processor can read information from, and write information to,the storage medium. In the alternative, the storage medium may beintegral to the processor. The processor and the storage medium mayreside in an ASIC. The ASIC may reside in a user terminal. In thealternative, the processor and the storage medium may reside as discretecomponents in a user terminal.

In one or more exemplary embodiments, the functions described may beimplemented in hardware, software, firmware, or any combination thereof.If implemented in software, the functions may be stored on, transmittedover or resulting analysis/calculation data output as one or moreinstructions, code or other information on a computer-readable medium.Computer-readable media includes both computer storage media andcommunication media including any medium that facilitates transfer of acomputer program from one place to another. A storage media may be anyavailable media that can be accessed by a computer. By way of example,and not limitation, such computer-readable media can comprise RAM, ROM,EEPROM, CD-ROM or other optical disk storage, magnetic disk storage orother magnetic storage devices, or any other medium that can be used tocarry or store desired program code in the form of instructions or datastructures and that can be accessed by a computer. The memory storagecan also be rotating magnetic hard disk drives, optical disk drives, orflash memory based storage drives or other such solid state, magnetic,or optical storage devices.

Also, any connection is properly termed a computer-readable medium. Forexample, if the software is transmitted from a website, server, or otherremote source using a coaxial cable, fiber optic cable, twisted pair,digital subscriber line (DSL), or wireless technologies such asinfrared, radio, and microwave, then the coaxial cable, fiber opticcable, twisted pair, DSL, or wireless technologies such as infrared,radio, and microwave are included in the definition of medium. Disk anddisc, as used herein, includes compact disc (CD), laser disc, opticaldisc, digital versatile disc (DVD), floppy disk and Blu-ray disc wheredisks usually reproduce data magnetically, while discs reproduce dataoptically with lasers. Combinations of the above should also be includedwithin the scope of computer-readable media.

Operations as described herein can be carried out on or over a website.The website can be operated on a server computer, or operated locally,e.g., by being downloaded to the client computer, or operated via aserver farm. The website can be accessed over a mobile phone or a PDA,or on any other client. The website can use HTML code in any form, e.g.,MHTML, or XML, and via any form such as cascading style sheets (“CSS”)or other.

Also, the inventors hereof intend that only those claims which use thewords “means for” are to be interpreted under 35 USC 112, sixthparagraph. Moreover, no limitations from the specification are intendedto be read into any claims, unless those limitations are expresslyincluded in the claims. The computers described herein may be any kindof computer, either general purpose, or some specific purpose computersuch as a workstation. The programs may be written in C, or Java, Brewor any other programming language. The programs may be resident on astorage medium, e.g., magnetic or optical, e.g. the computer hard drive,a removable disk or media such as a memory stick or SD media, or otherremovable medium. The programs may also be run over a network, forexample, with a server or other machine sending signals to the localmachine, which allows the local machine to carry out the operationsdescribed herein.

It is also noted that all features, elements, components, functions,acts and steps described with respect to any embodiment provided hereinare intended to be freely combinable and substitutable with those fromany other embodiment. If a certain feature, element, component,function, or step is described with respect to only one embodiment, thenit should be understood that that feature, element, component, function,or step can be used with every other embodiment described herein unlessexplicitly stated otherwise. This paragraph therefore serves asantecedent basis and written support for the introduction of claims, atany time, that combine features, elements, components, functions, andacts or steps from different embodiments, or that substitute features,elements, components, functions, and acts or steps from one embodimentwith those of another, even if the following description does notexplicitly state, in a particular instance, that such combinations orsubstitutions are possible. It is explicitly acknowledged that expressrecitation of every possible combination and substitution is overlyburdensome, especially given that the permissibility of each and everysuch combination and substitution will be readily recognized by those ofordinary skill in the art.

In some instances entities are described herein as being coupled toother entities. It should be understood that the terms “interfit”,“coupled” or “connected” (or any of these forms) may be usedinterchangeably herein and are generic to the direct coupling of twoentities (without any non-negligible, e.g., parasitic, interveningentities) and the indirect coupling of two entities (with one or morenon-negligible intervening entities). Where entities are shown as beingdirectly coupled together, or described as coupled together withoutdescription of any intervening entity, it should be understood thatthose entities can be indirectly coupled together as well unless thecontext clearly dictates otherwise.

Reference to a singular item includes the possibility that there are aplurality of the same items present. More specifically, as used hereinand in the appended claims, the singular forms “a,” “an,” “said,” and“the” include plural referents unless specifically stated otherwise. Inother words, use of the articles allow for “at least one” of the subjectitem in the description above as well as the claims below.

It is further noted that the claims may be drafted to exclude anyoptional element (e.g., elements designated as such by descriptionherein a “typical,” that “can” or “may” be used, etc.). Accordingly,this statement is intended to serve as antecedent basis for use of suchexclusive terminology as “solely,” “only” and the like in connectionwith the recitation of claim elements, or other use of a “negative”claim limitation language. Without the use of such exclusiveterminology, the term “comprising” in the claims shall allow for theinclusion of any additional element—irrespective of whether a givennumber of elements are enumerated in the claim, or the addition of afeature could be regarded as transforming the nature of an element setforth in the claims. Yet, it is contemplated that any such “comprising”term in the claims may be amended to exclusive-type “consisting”language. Also, except as specifically defined herein, all technical andscientific terms used herein are to be given as broad a commonlyunderstood meaning to those skilled in the art as possible whilemaintaining claim validity.

While the embodiments are susceptible to various modifications andalternative forms, specific examples thereof have been shown in thedrawings and are herein described in detail. It should be understood,however, that these embodiments are not to be limited to the particularform disclosed, but to the contrary, these embodiments are to cover allmodifications, equivalents, and alternatives falling within the spiritof the disclosure. Furthermore, any features, functions, acts, steps, orelements of the embodiments may be recited in or added to the claims, aswell as negative limitations (as referenced above, or otherwise) thatdefine the inventive scope of the claims by features, functions, steps,or elements that are not within that scope. Thus, the breadth of theinventive variations or invention embodiments are not to be limited tothe examples provided, but only by the scope of the following claimlanguage.

That being said, we claim:
 1. A system for characterizing a sample, thesystem comprising: a first optical path configured to propagate aninterrogating optical beam to the sample, the interrogating beamcomprising a plurality of optical pulses, two consecutive pulses of theinterrogating beam being spaced apart by a period (T), the period (T)corresponding to a repetition rate of the interrogating optical beam,said interrogating optical beam generating second harmonic generatedlight from the sample; an optical pump source outputting pump radiation;a detector configured to sample intensities of the generated secondharmonic light at at least two temporally separated sample times, thetwo sample times occurring after charging the sample at a sample site byoptically pumping; a controllable optical delay system configured tointroduce a variable time delay (Δ) between at least one pulse from theinterrogating beam and a reference time with respect to said chargingthe sample, the variable time delay Δ being shorter than the period (T)of the pulses from the interrogating beam, said controllable opticaldelay system configured to vary and control the duration of the timedelay; and a processing and control module configured to obtain acharacteristic of the sample based on intensities of said secondharmonic generated light at the at least two sample times.
 2. The systemof claim 1, further comprising a first optical shutter disposed in thefirst optical path, the first optical shutter configured to block one ormore pulses from the interrogating optical beam.
 3. The system of claim2, wherein the first optical shutter comprises at least one of amechanical shutter, an electro-optic shutter, a Kerr cell and a Pockelscell.
 4. The system of claim 2, wherein the processing and controlmodule is configured to control the first optical shutter.
 5. The systemof claim 1, further comprising a second optical path configured topropagate pump radiation that provides said optical pumping.
 6. Thesystem of claim 5, further comprising a second optical shutter disposedin the second optical path, the second optical shutter configured toblock the pump radiation. 7.-48. (canceled)
 49. A method of opticalinterrogation of a sample material, the method comprising: charging thematerial at a sample site with radiation to a first charging level;obtaining charge state decay measurements at the sample site in lessthan one second by directing an interrogating optical beam at the samplesite at multiple time points to provide decay curve data; anddetermining a characteristic of the decay curve data to determine aparameter of the sample material.
 50. The method of claim 49, whereindetermining a characteristic of the decay curve data includesdetermining a shape of the decay curve data.
 51. The method of claim 49,wherein the parameter is selected from at least one of: a charge carrierdensity, a trap charge density, an occupied trap density, a carrierinjection threshold energy, a charge carrier lifetime, a chargeaccumulation time, and trapping cross-section.
 52. The method of claim49, further comprising obtaining charge state decay measurements atanother sample site at multiple time points to provide the decay curvedata.
 53. The method of claim 52, further comprising charging thematerial at another sample site with radiation to a second charginglevel.
 54. The method of claim 53, wherein the first and the secondcharging level are same.
 55. The method of claim 53, wherein the firstand the second charging level are different. 56.-98. (canceled)
 99. Asystem for optical interrogation of a sample material, the systemcomprising: an optical source capable of charging the material at asample site with radiation to a first charging level; a detection systemcapable of obtaining charge state decay measurements at the sample siteat multiple time points temporally separated by less than 1 second toprovide a decay curve data; and a processor capable of determining acharacteristic of the decay curve data to determine a parameter of thesample material.
 100. The system of claim 99, wherein the detectionsystem is capable of obtaining charge state decay measurements at thesample site at multiple time points temporally separated by less than 50milliseconds to provide a decay curve data.
 101. The system of claim 99,wherein the detection system is capable of obtaining charge state decaymeasurements at the sample site at multiple time points temporallyseparated by less than 1 millisecond to provide a decay curve data. 102.The system of claim 99, wherein the detection system is capable ofobtaining charge state decay measurements at the sample site at multipletime points temporally separated by less than 500 microseconds toprovide a decay curve data.
 103. The system of claim 99, wherein theoptical source comprises at least one of: a UV flash lamp, a laser and apulsed laser.
 104. The system of claim 99, wherein the detection systemcomprises a probe beam; and a detector capable of detecting a secondharmonic generation (SHG) signal associated with the probe beam. 105.The system of claim 104, wherein the probe beam is emitted by a pulsedlaser. 106.-120. (canceled)